Characterization of Precursors of Antibody Forming Cells - a) Affinity of Receptors: Isolation of precursors of specific antibody forming cells and determination of the relationship between affinity of antibody formed and avidity of the receptors on the isolated antigen binding precursors. b) Class of Receptors: Isolation of precursors with receptors of different immunoglobulin classes and determination of the class of intracellular immunoglobulin in these precursors and in their descendants. c) T Cell Regulation of Precursor Differentiation: Determination of the influence of T cells on the regulation of affinity and class of antibody produced by descendants of isolated antibody forming cell precursors. T-Cell and B-Cell Subsets: isolation and characterization of subsets of T cells and B cells and definition of their function in humoral and cell mediated immunity. Idiotypes and Allotypes: further investigation of the expression of V sub H (idiotype) and C sub H (allotype) genes on individual Ig molecules in doubly heterozygous mice. Continue to Develop and Study Immunoglobulin Allotype (C sub H) Congenic Strains: a) New congenic strain development; b) Maintenance and continued genetic testing of existing congenic strains; c) Look for specific genetic differences between BAB/20 and Potter's BALB/c-2/20. Continuation of Studies to Determine Whether the Ir-1 (Immune Response-1) Locus is Expressed on T and/or B Cells.